During the metamorphic transition from aquatic to terrestrial life in the frog, a corresponding shift in larval swimming to adult-like hindlimb locomotion are functionally mature no later than midlarval stages, or about a year before hindlimb locomotor behavior occurs. I have shown in electrophysiological studies in vitro that the spinal circuits for hindlimb locomotor behavior commences. This fact and the results of other experiments I have performed suggest that hindlimb locomotor activity is suppressed in the larva, probably by supraspinal structures. A second change associated with metamorphosis is a precipitous decline in spontaneous activity of locomotor circuits in vitro (fictive locomotion). It is hypothesized that the change in locomotor topography and gross levels of fictive locomotion during metamorphosis are the result of the dramatic development of sensorimotor structures (e.g., cerebellum and optic tectum) associated with this period of development. It is specifically hypothesized that spinal hindlimb locomotor circuits are tonically inhibited in both larval and adult life and are disinhibited in response to particular sensory stimuli. The relative increase in use of the hindlimbs in locomotion and the decline of fictive locomotion in vitro are both assumed to occur because changes in peripheral sensory structures and associated changes in sensorimotor integration increase the specificity and complexity of the adequate stimuli for release of locomotion. This hypothesis is consistent with a diverse set of manipulations that elicit precocial hindlimb use, the behavior of adult anura, concepts of locomotor development as derived from developing chicks and mammals, anatomical studies of metamorphic changes in the frog's brain and mechanisms of locomotor control in invertebrates. Because of the unique advantages of the bullfrog larva as a model system this hypothesis is easily testable, and the experiments should add to our understanding of vertebrate locomotor development and organization, help bridge the gap between invertebrate and vertebrate studies and provide a basis for more sophisticated analyses of motor development and control. Tests of alternative hypotheses are also included.